Dice the pepper and onion. Heat 1 tablespoon of oil in a skillet.
Add the pepper and onion to the skillet and cook until vegetables have softened, about 5 minutes. Stir in the garlic and cook 1 minute more.
Remove the vegetables to the slow cooker and add the chicken broth, potatoes, and cajun seasoning.
Cover and cook on high for 3 hours or until potatoes are fork tender.
Add the flour and milk to a mason jar and close tightly. Shake well to combine. Alternately, whisk the flour and milk together in a small bowl.
Pour flour mixture into the soup and stir to combine. Add the cream, evaporated milk, and grated cheddar to the soup and stir.
Cover and cook on high for 30 more minutes.
Use a potato masher to mash the potatoes to reach the desired consistency for the soup. Taste and add additional cajun seasoning or salt and pepper, if needed.
After the soup is your desired consistency, heat the remaining tablespoon of oil in a large skillet. Slice the sausage into ½ inch thick rounds and add to the skillet. Brown on both sides.
Stir the browned sausage into the soup.
Ladle hot soup into bowls and garnish with cheddar and green onion, if desired.